by C.D. Feng, senior manager of sensor technology, Emerson Process Management, Rosemount Analytical
Ground Loop is the most common and yet dreadful phrase given to field service engineers when electrochemistry based sensors (pH, DO, chlorine, etc.) misbehave. The phrase is well established in the electrical/electronic world, but very heavy headed in the field of industrial electrochemical sensor applications. As a head of technology at Rosemount Analytical, I would like to establish a steady foundation for this mostly common used phrase: Ground Loop.
In order to establish a steady foundation, we can’t go fast, we have to go slow, one step at a time, however illogical, to the current business world. We also have to start from the ground, definitely not from the iCloud.
What I am going to start with is physics – high school physics – combined with the term “equivalent circuit” from advanced electrochemistry. You, however, do not need to dive too much into the advanced electrochemistry to find out exactly how we get the equivalent circuit. All you need to do is to remember that an electrochemistry-based sensor can be represented by a certain arrangement of various electrical components, such as resistors, capacitors, batteries and so on.
Let’s start with a pH sensor. An industrial pH sensor comprises two parts, one is the glass electrode, and the other is the reference electrode. The equivalent circuit of a glass electrode is like this:
Where Eg is the potential generated on the glass electrode and is part of the voltage signal you are trying to measure, and Rg is the resistance of the glass electrode. The blue end is the solution end, and the red end is the wire lead end.
The reference electrode has a very similar equivalent circuit as that of the glass electrode:
Where Er is the potential of the reference electrode, and Rr is the resistance of the reference junction (physically, the junction is a porous material made of either ceramics or Teflon). The same thing applies here – the blue end is the solution end, and the red end is the wire lead end.
What about a piece of metal stick in a solution?
Simple, all you have is a battery. (That’s basically what you get when you stick a piece of metal into a solution – maybe not a good battery, but definitely a battery.)
Is there an equivalent circuit for just solution? The answer is yes, and the equivalent circuit for solution is just a resistor:
Now let’s have a little bit fun – let’s see what the equivalent circuit looks like when you place a pH sensor in a sample solution:
This is enough fun we have had this time.
Next time, I am going to show you the first Ground Loop scenario using these circuits.
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